Implanted markers have been used in radiotherapy for x-ray based target localization. The projected marker position in a series of cone-beam CT (CBCT) projections may be used to estimate a three dimensional target trajectory during the CBCT acquisition. Also, in radiotherapy, precise delivery of the treatment dose is crucial in order to maximize the ratio between tumour dose and normal tissue dose. To achieve this goal x-ray visible markers may be implanted in or near a tumour. It allows use of the projected marker trajectory in x-ray images for image-guided radiotherapy (IGRT).
However, Applicant of the subject application determines that reliable segmentation of the markers in the x-ray images may be difficult if the markers are irregularly shaped and/or viewed from varying directions such as in a cone-beam CT scan.